The invention relates to a guiding device for guiding a moveable machine element of a machine and also to a machine having such a guiding device.
In machines such as machine tools, production machines and/or robots, a moveable machine element such as a guiding carriage frequently has to be displaced in a position-regulated manner along a guiding device which may be in the form of a guiding track, for example. FIG. 1 schematically shows a commercial machine tool, where, for reasons of clarity, only those elements which are necessary to understand the invention are shown. A moveable machine element 1, which is designed in the exemplary embodiment in the commercial machine in the form of a guiding carriage, is guided by a guiding device 3, which is provided in the exemplary embodiment in the form of a guiding track. The machine element 1 is equipped with a drive, for example a linear drive, which is not shown for reasons of clarity and is used by an open-loop and/or closed-loop control system to displace the machine element 1 in a position-regulated manner along the guiding device 3. The guiding device 3 is connected in a mechanically fixed manner to a machine bed 4 which stands on a floor 5.
A measuring scale 6, which may be, for example, in the form of a strip having a regular graduation, is connected in a mechanically fixed manner to and along the guiding device 3 in order to determine the position of the machine element 1. The measuring scale 6 has, for example, a “graduation” in the form of lines, which graduation can be read by a reading head 2 connected to the machine element 1. The measuring scale 6, in conjunction with the reading head 2, therefore permits an exact determination of the position of the machine element 1.
In a commercial machine, further fixtures, for example in the form of a spindle with tools, which can be displaced along the guiding device 3 using the machine element 1, are provided on the latter, which is in the form of a “guiding carriage” in the exemplary embodiment. In a machine of this type, the workpiece to be machined is often anchored directly to the floor. The machine element 1 and the fixtures attached thereto form a “tool carriage”. If desired, a tool carriage can have multiple guiding carriages rather than just one guiding carriage. In machines such as machine tools, production machines and/or robots, the machine carriages must be positioned with a high degree of precision and at high speed, while maintaining prescribed displacement profiles. The prescribed displacement profiles include speed changes which necessitate high acceleration forces. The quick change in the acceleration forces causes the mechanical system of the machine, in particular the machine bed 4, to oscillate. The machine bed 4 is often stressed very strongly by the reaction forces of the movement guidance. The influence of the acceleration forces of the tool carriage causes the machine bed to give way in the opposite direction. In the process, it is frequently deformed and/or begins to oscillate. Even if the machine element 1 can be positioned exactly in relation to the machine bed 4, the machine bed 4 itself still oscillates with respect to the floor 5. If the machine is a portal milling machine, for example, in which the workpiece to be machined is fixedly anchored on the floor, the oscillations of the machine bed 4 can be seen from the workpiece, even if the machine element 1 maintains exactly the prescribed position in relation to the machine bed 4.
The oscillations of the machine bed 4 frequently have such an unfavorable effect that the closed-loop control characteristics of the movement guidance of the machine element 1 are strongly compromised and not even a movement guidance of the machine element 1 in relation to the machine bed 4 that is sufficiently precise can be achieved. In that case the oscillations of the machine bed 4 occurring during the movement guidance have a doubly disadvantageous effect.
The prior art strives to suppress undesired oscillations of the machine bed by means of a correspondingly rigid and heavy design of the machine bed. The machine bed, however, becomes heavy, complex and expensive on account of this.
If a direct linear drive is used as the driving system for the machine element, it is possible to separate the reaction surface of the motor from the machine bed. This fact is disclosed by the laid-open specification DE 198 10 996 A1. The document proposes decoupling of the motor reaction surface from the machine bed, with the reference point of the axis control remaining on the machine bed. The document proposes to anchor the motor reaction surface with respect to the machine bed such that it can move. This causes the reaction force which arises as a direct consequence of the acceleration operations to be absorbed in the motor reaction surface of moveable design and in this manner to be kept away from the machine bed. The machine bed therefore remains at rest even given severe changes in speed of the tool carriage, which significantly improves the accuracy of the movement guidance of the tool carriage. The reference point for the determination of position remains here unchanged on the machine bed for the closed-loop control, for example. Feed drives designed according to this technical teaching are used in machine tools for example in which the tool carriage is subjected to high accelerations. It has been shown, however, that the necessary moveable anchoring of the motor reaction surface of the linear direct drive results in significant costs and complexity. They are therefore not used especially as a device for avoiding oscillations in machine beds of cost-effective and light-weight construction.
The laid-open specification DE 198 10 996 A1 proposes to decouple the motor reaction surface from the machine bed in order to improve the movement guidance in a machine having a machine bed which is able to oscillate.
It is known from the laid-open specification WO 91/16594 to integrate a position measurement system into a rolling element guiding means. The measuring scale of the position measurement system is here fixedly anchored to the guide track of the rolling element guiding means.
The journal “Werkstatt und Betrieb” [Workshop and Operations], page 160 to 164, year 133 (2000)6, Carl Hansa Verlag, Munich, discloses a guiding device located on a first portal for guiding a moveable machine element, wherein a measuring scale is attached to a second portal which is physically separate from the first portal. The measuring scale and the guiding device are physically separate from one another on account of the spacing between the two portals. The physically wide and relatively irregular spacing between the two portals entails that the spacing between measuring scale and the reading head of the machine element is subject to fluctuations. The measurement system, however, reacts with great sensitivity to change in the spacing between measuring scale and guiding device because, for example, the graduation marked on the measuring scale (a graduation may be provided for example in the form of lines marked on the measuring scale) can no longer be read off exactly by the reading head. This has a very strongly negative effect on the positioning accuracy of the machine element.